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CELL REPRODUCTION Cells grow but size is limited. A. Too large, not enough DNA to control cell functions B. Too large, not enough surface area for volume Cells divide in order to survive. Cell cycle of growth and division has two stages: A. mitosis B. cytokinesis. The Cell Cycle Interphase: The period of growth between cell divisions. G-1 cell grows larger, proteins and organelles are made. S DNA is made and chromosomes replicate. G-2 organelles, molecules needed for division are produced. Mitosis: The division of the cell nucleus. Prophase: Chromatin coils, thickens. Chromosomes become visible. Centrioles move to poles. Chromatids attach to spindle. Nuclear membrane and nucleolus breakdown. Metaphase: Sister chromatids line up along equator. Anaphase: Centromeres split and sister chromatids move to poles. Telophase: Chromosomes uncoil spindle breaks down, nuclear envelope and nucleolus reform. Cytokinesis: Division of the cytoplasm. Animals: membrane pinches in. Plants: cell plate forms. Prokaryotes have no organized nucleus so they do not go through mitosis. Eukaryotes control their cell cycle with enzymes (special proteins) known as regulatory proteins or cyclins. Internal regulators control mitosis stages External regulators control cell cycle timing Human Embryology Multicellular organisms grow bigger through mitosis and cytokinesis. Cells will become specialized to perform certain functions. Development begins with fertilization of egg cell by sperm cell. The fertilized egg is called a zygote. The zygote divides to form 2 cells. Division occurs again to give 4 cells. Repeated divisions result in a ball of 64 cells called a morula. The cells in a morula are totipotent stem cells. They are undifferentiated cells that have the potential to become any kind of cell in the body. Continued cell division eventually results in a hollow ball of cells called a blastula. As cells continue to divide and move, the blastula folds in on itself and becomes a gastrula. The cells are now becoming less able to develop into any other kind of cell and are called pluripotent. As development continues, three distinct kinds of cells emerge and are located in regions called germ layers. Endoderm : The innermost layer, becomes the lining of the digestive tract and much of the respiratory system. Mesoderm : The middle layer, becomes muscle tissue and much of the circulatory, reproductive and excretory systems. Ectoderm : The outermost layer, becomes the sense organs, nerves, and the outer layer of the skin. Apoptosis : programmed cell death required for proper development; also rids the body of defective or diseased cells. Pluripotent stem cells will differentiate into 4 major branches: 1. hematopoietic stem cells erythrocytes basophils lymphocytes thrombocytes neutrophils monocytes 2. mesenchymal stem cells bone tendon cartilage muscle adipose marrow 3. endodermal stem cells esophagus pancreas liver intestines stomach lungs 4. neural stem cells neurons oligodendrocytes glial cells astrocytes These four major branches will differentiate into 210 types of specialized somatic (body) cells About 8 weeks after fertilization, The embryo becomes a fetus containing pluripotent fetal stem cells Stem cells from the placenta and adults have limited potential and are called multipotent stem cells.